Solid-state semiconductor memories (ROM's) are available in the art and have numerous applications in electronic systems and computers as random logic elements, character generators, etc. It is a characteristic of a conventional ROM that its memory content (encoded state) is determined during the device fabrication process. An alteration in the content of the memory requires expensive re-design of the device in which it is incorporated which makes ROM's impractical for incorporation in equipment with small production runs. For this reason, the programmable read-only memory (pROM) was developed. Commercially available versions of this memory are generally shipped to the user in an unencoded state. The memory content is then coded, or programmed, at the user's location which allows the user to make the necessary changes on site. The pROM is also called a "write-once" ROM because once a bit is programmed it cannot be erased.
A typical pROM incorporates an x-y matrix of conductors, the cross points of which are connected together by diodes in series with resistive fuses such as the thin film pROM shown in U.S. Pat. No. 3,245,051. Each x-line of the pROM constitutes an address, and the y-lines together constitute an output word. If no fuses have been opened, the output word will consist of all "1's". In other words, if a "read" voltage is impressed on address line X1 (assuming for example, that the pROM has four address lines and five output bits) all output bits (y1-y5) will have a voltage impressed on them. If during programming of this memory, sufficient current (the "write" current) was allowed to flow through certain selected fuses associated with address line X1, those fuses would be blown into the open or non-conducting state. Subsequent application of a read signal to address X1 would result in an output word in which the only bits with 1's in them correspond to fuses which have not been blown. The function of the diodes is to provide a solution to the well known problem of "sneak-paths" associated with matrix memory schemes. These diodes provide effective isolation between adjacent bits so that an output bit which is programmed as a "0" will not receive an erroneous "1" signal through alternate conduction paths. The pROM described in the aforementioned patent, it should be noted, is not capable of operation in a transposed mode (i.e. applying an input signal on the output lines to generate a signal or word on the input lines). Further, the fuse when blown open, will be accomplished over a relatively extended period of time which could either allow the fuse to close after extended usage or the fuse may not be cleanly opened.
The established technology of silicon integrated circuits is well suited for the fabrication of pROM's since the matrix array of diodes can be formed by conventional diffusion processes, and the fuses can be formed by vapor deposition of appropriate fuse material and many of the available pROM's are fabricated by that technology. U.S. Pat. No. 3,818,452, shows a pROM which utilizes silicon technology.
However, it would be advantageous if a pROM could be commercially fabricated using thin film technology instead of the silicon technology described hereinabove, the fabrication cost of the resultant pROM being reduced.
It would also be desirable if the pROM, when fabricated, has the capability of being transposable which allows the role of address and word lines to be interchanged, i.e. the roles of the rows and columns can be reversed providing a pROM having at least dual operative capabilities. Further, it would be advantageous if the pROM device was of the type that allows the associated fuse to be rapidly and cleanly blown when the appropriate current is applied thereto.